454 MANUAL OF THE MOLLTJSCA. 



Suh-genera. Hemicardium (Cardissa) Cuvier. C. hemicardium, 

 PI. XIX.. Fig. 3. Shell depressed, posterior slope flat, valves 

 prominently keeled. 



Lithocardium aviculare, PI. XVIII., Fig. 17. Shell triangular, 

 keeled; anterior side very short; hiiigo-teeth 1.2, directed 

 backwards ; posterior laterals 2.1; anterior muscular pit minute, 

 posterior improssion large, remote from the hinge. L. cymbulare, 

 Lam,, exhibits slight indications of a byssal sinus in the front 

 margins of the valves. Fossil, Eocme, France. These shells 

 present considerable resemblance tc Tridacna. 



Serripes (groenlandicus) Beck. Hinge edentulous. Arctic 

 Seas, from C. Parry to Sea of Kara; fossil in the Norwich Crag. 



Fig. 253. C. Iceviuscidum, Eichw. (after Middendorff). 



Adacna, Eichwald. C. edentulum, PI. XIX., Fig. 4. (Acardo, 

 Sw. not Brug. Pholadomya, Ag. and Mid. not Sby.) Shell com- 

 pressed, gaping behind, thin, nearly edentulous; pallial line 

 sinuated. Animal with the foot (/) compressed; siphons (s) 

 elongated, united nearly to the end, plain. Bistrihution, 8 species. 

 Aral, Caspian, Azof, Black Sea, and the embouchures of the 

 Wolga, Dniester, Dnieper, and Don; burrowing in mud. G. 

 Caspicum (Monodacna, Eichw.) has a single hinge-tooth, and 

 C. trigonoides (Didacna, E.) rudiments of two teeth. The 

 siphonal inflection varies in amount. 



Distribution, 200 species. World-wide ; from the sea-shore to 

 140 fathoms. Gregarious on sands and sandy mud. 



Fossil, 330 species. Upper Silurian — . Patagonia — 

 Southern India. 



C. Hillanumy Sby. (Protocardium, Beyr.), is the type of a 

 small group in which the sides are concentrically farrowed, the 

 posterior slope radiately striated; the palliai lino is slighxly 

 sinuated. Jura — Chalk ; Europe, India. 



CoNOCARDnjM, Bronn. 



Synonyms, Lychas, Stein. Pleurorhynchus, Ph. Lur-ulo 

 cardium. Mlinst^r. 



